the difference in nuance

The first one means he is worrying about her right now, that's what he's thinking about. The second one means he is worried, but he might be thinking of something else right now, like the washing up, or mending a fuse.

Quote kodokan:


Sentence 1 means that he is presently, actively worried about her, but it is a short term condition and he might not be worrying about her in the near future.

Sentence 2 means that the worry condition is presently permanent and won't change unless there is a significant change of circumstances.

For example, mothers are perpetually worried about their children who might be living away at school. However, they are usually said to be worrying about them only if there is some specific issue that has arisen, like they are ill for a couple of days.

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